1. Field of the Invention
The present invention relates to the protection of aircraft structures from the effects of explosions.
2. Discussion of Prior Art
There have been a sufficient number of aircraft bombings, suspected bombings and foiled bombings recently for there to be dawning in the consciousness of those who travel frequently by air the worry that their lives may soon be more at risk from deliberate damage to, as opposed to accidental failure of, the complex structures in which they travel.
The precise sequence of events following the detonation of a bomb on board an aircraft depends upon the location of the bomb and the size and design of the aircraft. However, certain features are common to most events which have been investigated.
It has been observed that aircraft can survive the detonation of bombs on board, provided certain features are present. A bomb placed near the outer skin of the aircraft will, most probably, blow a hole in the skin and cause explosive decompression. However, it has often been the case that the aircraft can still land normally. The same seems to be the case even for bombs which have exploded in the luggage compartments of the older types of aircraft. Luggage in these aircraft is contained in cargo nets, rather than the standard international luggage containers that the more modern wide-bodied aircraft use. Indeed, the accepted minimum risk position for a bomb discovered on board any type of aircraft is to place it by a door, with the interior side of the bomb tamped with cushions. The reason for choosing this position is because of the high survivability rate observed in cases of bombs planted against the aircraft hull in regions that passengers or cleaning staff have access to, which rarely destroy vital electronics or hydraulic systems and do not always damage significant load-bearing members so as to weaken the overall structure and because of recent cases of aircraft surviving massive losses of skin around regularly shaped fatigue failures in the hull.
However, in cases where the bomb is placed in a position not adjacent to the outer skin, severe, often fatal, damage can be caused.
Passenger cabin floors are relatively light structures laid on load-bearing beams. These floor beams may be tension load carriers as in the case of Boeing 747s, because of the non-circular nature of the aircraft cross-section. It is thus very possible for a bomb to damage the floor beams and, as a result, load the aircraft skin remotely from the site of the explosion asymmetrically both before and after the hull is breached by the bomb. Blast may also travel significant distances within the hollow and open channels in the aircraft structure to emerge at points well removed from the site of the bomb to cause skin and stringer rupture at several locations on the aircraft skin. Blast may also emerge into the relatively large free space of the passenger cabin and, because of the presence of rigid and substantial structures such as galleys or toilets, reflect on to the inside of the aircraft skin remote from the bomb and do unexpected damage there.
Blast emerging from the aircraft skin either close to or remote from the site of the bomb tends to tear irregularly shaped holes (as opposed to the cases of the more regular skin failure due to fatigue or, say, loss of a hold door). Regularly shaped holes are less often associated with catastrophic failure of the aircraft than irregularly shaped ones. Irregular holes tend to suffer enlarging and further skin damage due to the outrush of cabin air and slipstream effects.
It is possible that some modern wide-bodied aircraft may be more vulnerable to bombs of a similar size to those that have not always caused crashes of smaller aircraft. One theory is that, since the wide bodied aircraft employ a skin sheeting alloy which is only slightly thicker than that used in very much smaller aircraft, the latter are much stiffer structures than the former ones. Consequently, the smaller ones can withstand greater relative damage to their skin and stringers than the larger ones.